Development and Application of Drilling Fluid System for Reservoir Protection of Deep Gas Reservoir in Nanpu Offshore Area

Offshore area of Nanpu Sag is a small continental fault depression located at northern Bohai Bay Basin, with rich oil and gas resources. The reservoir protection is rather difficult due to the complicated geological conditions and the poor understanding to the mechanism of reservoir damage in this area, which results in the occurrence of high-yield well or dry well within a small range with the same target layers, and with the similar situation in the gas production. The comparison and analysis of the recovered gas and oil permeability of core samples measured pre-and post-displacement with different fluids indicate that the water blocking damage in the gas reservoir is 30~50 percent higher than that of oil reservoir. On the basis of that, 180℃ high temperature polymeric micelle drilling fluid was developed under the guidance of reverse osmosis and water-lock prevention theory. Experimental results show that the membrane efficiency of the drilling fluid increases from 0.11 to 0.5, and the gas permeability recovery of core sample reaches 91%. The application of the drilling fluid in Well NPAX-C and another well shows that the drilling fluid can meet the needs of drilling the tight gas reservoir with water-based drilling fluid in Nanpu offshore area due to its perfect shale inhibition and gas reservoir protection performance. This study provides more reliable technical support to the efficient exploration of the tight gas reservoir in Nanpu Sag, and the technical thinking also can provide valuable reference and demonstrative effect on the exploration and development of the other areas with the similar tight gas reservoir.